Accompanied by a trend in favor of a small-sized and thinner-shaped electronic parts, circuits and electrodes (hereinafter, referred to as “electrode(s)”) used in such electronic parts are required to have a higher density and higher precision.
For connecting such an electrode to another, a method has been known which comprises using an anisotropically conductive adhesive, obtained by dispersing conductive particles such as carbon, nickel, and a metal-coated plastic in an insulating adhesive, or a film thereof and applying a pressure while heating to achieve electrical connection in a thickness direction of the adhesive (see, for example, Japanese Prov. Patent Publication No. 104007/1980).
However, in the anisotropically conductive adhesive containing conductive particles, each conductive particle must have a particle diameter not larger than the space between the adjacent electrodes (hereinafter, referred to as “space”), and further, at least one conductive particle is needed per electrode, thereby limiting the degree of density and precision. Specifically, when the particle diameter of the conductive particles is reduced for improving the probability of the conductive particles present on each fine-structured electrode, the particles are likely to suffer aggregation to cause short-circuiting between the electrodes. In contrast, when the particle diameter of the conductive particles is increased, the probability of the conductive particles present on each fine-structured electrode is disadvantageously lowered.
On the other hand, there has been a method for connecting electrodes without using conductive particles, which comprises applying a pressure onto electrodes having microscopically uneven surfaces so as to remove an unnecessary adhesive and achieve electrical connection thereby (see, for example, Japanese Prov. Patent Publication No. 262430/1985).
However, when no conductive particle is used, a problem arises such that the reliability of connection between a number of electrodes becomes insufficient because the method fails to deal with uneven heights of the electrodes.
Further, when the surface of the electrode on the wiring board having an insulating part and an electrode part is exposed, the electrode disadvantageously suffers corrosion. Therefore, the surface of the electrode is conventionally subjected to anticorrosion-treatment, using a chelating agent, benzotriazole, etc.
However, the anticorrosive agent used for the electrode surface has an insulating property, and therefore it must be removed by washing before connecting electrodes, thereby causing an oxidation in the electrodes immediately before the connection and making the process more complicating.
In the present invention, there are provided a method for electrically connecting electrodes, which can deal with a trend of a higher density and higher precision of the electrodes without using conventional conductive particles, a surface-treated wiring board used in the method, which can prevent the electrode surface from suffering corrosion, an adhesive film which can electrically connect electrodes with a higher density and a higher precision even when the electrodes have uneven heights, and an electrodes-connected structure using them.